1. Field of the Invention
The present invention relates to an oil pump apparatus for supplying operating fluid to a power-assisting portion of a power steering apparatus of vehicles and the like. More particularly, the present invention relates to improvements of a bypass passage located between an inlet side of the pump mechanism portion and a flow control valve.
2. Description of the Prior Art
Oil pump apparatuses have been proposed for a power steering apparatus of vehicles and the like. In general, the conventional oil pump apparatus mainly consists of a pump mechanism portion and a flow control valve. The pump mechanism portion supplies operating fluid (i.e., oil) to a power-assisting portion of the power steering apparatus. The flow control valve maintains a flow rate of the operating fluid supplied to the power-assisting portion constant by draining part of the operating fluid to an inlet side of the pump mechanism portion as excess operating fluid.
An example of the oil pump apparatus is shown in a Japanese Utility Model No. 05-19594. As shown in FIG. 1, this oil pump apparatus includes a bypass hole 30 and a bypass passage 20 connecting with each other. The bypass hole 30 connects with the flow control valve 31 and the bypass passage 20 connects with the inlet side of the pump mechanism portion. The excess operating fluid is drained through the bypass hole 30 and the bypass passage 20.
The oil pump apparatus also includes an opening 150 of a reservoir passage connecting to a reservoir. The opening 150 is located in the connecting portion between the bypass hole 30 and the bypass passage 20.
In the conventional oil pump apparatus, a cross-sectional area of the bypass passage 20 widens in a side of the opening 150 of a reservoir passage, i.e., the center axis of the bypass passage 20 is placed offset from the center axis of the bypass hole 30 (shown by an eccentric distance d). In the configuration, since strong fluid stream (shown by arrows A) of the excess operating fluid is drained with causing negative pressure, the operating fluid is effectively led from the reservoir to the inlet side of the pump mechanism portion. As a result, enhanced is suction efficiency of the operating fluid supplied from the opening 150 of a reservoir passage, i.e., supercharging effect. Therefore, a width of the bypass passage 20 is designed to be as wide as possible in the side of the opening 150 in order to include almost of all area of the opening 150.
After gathering in the bypass passage 20, the excess operating fluid drained from the flow control valve 31 and the operating fluid sucked by the jet, i.e., strong stream A, of the excess operating fluid are led to an inlet port of the pump mechanism portion.
As described above, when the excess operating fluid is drained from the bypass hole 30 to the bypass passage 20, the stream of the excess operating fluid spouts with high pressure as the jet A. The jet A dashes against an inner surface of the bypass passage 20 near the bypass hole 30, so as to possibly cause cavitation damages, i.e., erosion. In addition, since the cavitation removes tiny broken pieces from the inner surface of the bypass passage 20, the tiny broken pieces enter in the pump mechanism portion, so as to deteriorate quality of the pump mechanism portion.